For the purification of exhaust gas of an internal combustion engine, various catalytic converter devices may be arranged in the exhaust tract of the internal combustion engine. For the removal of nitrogen oxides from the exhaust gas for temporary adsorption and subsequent conversion into nitrogen oxides, use is made of nitrogen oxide storage catalytic converters (also referred to as lean NOx traps, LNT). During lean-burn operation of an internal combustion engine, that is to say during operation with an air/fuel mixture in which an excess of air and thus oxygen is present, nitrogen oxides that are generated can be stored in an LNT; for this purpose, the LNT oxidizes the nitrogen monoxide (NO) contained in the lean exhaust gas to form nitrogen dioxide (NO2), and subsequently stores this in the form of nitrates. With increasing nitrogen oxide loading of the LNT, the storage capacity thereof progressively decreases.
To restore the storage capacity of the LNT, the LNT may be regenerated. During the regeneration, the stored nitrogen oxides are desorbed again and are reduced, on catalytically active components of the LNT and with the aid of the rich exhaust-gas constituents (CO, HC), to form nitrogen. For this purpose, the exhaust gas is enriched, for example by way of operation of the internal combustion engine with a rich mixture or a corresponding increase of the fuel injection quantity in the engine and a reduction of the fresh-air supply. Here, a sub-stoichiometric ratio of oxygen to fuel is generated. The ratio may also be expressed by a lambda value, which is then less than 1. Said sub-stoichiometric ratio may also, aside from rich operation of the internal combustion engine, be produced by injection of fuel into the exhaust tract. Here, it is primarily nitrogen oxides that are removed, for which reason this is also referred to as a DeNOx purge.
Furthermore, nitrogen oxides may be removed from the exhaust gas by catalytic converters for selective catalytic reduction (SCR catalytic converter). Here, the nitrogen oxides are reduced to form nitrogen with the aid of a reducing agent, generally ammonia, which is introduced in the form of an aqueous urea solution into the exhaust tract and is stored in the SCR catalytic converter.
Operation of the catalytic converters for the reduction of nitrogen oxides may be carefully controlled. Control variables such as emissions, fuel economy, vibrations of the corresponding motor vehicle, and reducing agent consumption may be weighed against one another. Here, the operation of the catalytic converter devices and of the internal combustion engine are also controlled with mutual dependency. Furthermore, vibrations of the motor vehicle and the emissions of carbon dioxide are to be kept as low as possible.
In the case of fixed settings of the internal combustion engine and of the exhaust-gas aftertreatment devices, the pollutant emissions vary considerably in a manner dependent on factors such as driving style and operating conditions. High nitrogen oxide emissions could duly be remedied by constant introduction of reducing agent, but this can result in undesired ammonia slippage. Furthermore, with the high consumption, reducing agent would be wasted. It would alternatively be possible for the LNT to be regenerated more frequently, but frequent regeneration of the LNT adversely affects fuel consumption.
The inventors herein have recognized the above issues and provide an approach to regulate the nitrogen oxide emissions of an internal combustion engine while realizing efficient consumption of reducing agent and fuel. In one example, a method for controlling nitrogen oxide emissions in exhaust gas of an internal combustion engine, in an exhaust tract of which there are arranged at least one nitrogen oxide storage catalytic converter (LNT) and at least one catalytic converter for selective catalytic reduction (SCR), at least one nitrogen oxide sensor, an introduction device arranged upstream of the SCR for a reducing agent, and a control device, is provided. The method includes measuring or estimating nitrogen oxide values in an exhaust tailpipe region and detecting an upward deviation of the nitrogen oxide values in the exhaust tailpipe region from a setpoint value. The method further includes, responsive to detecting the upward deviation, actuating the introduction device in order to provide a first correction of the nitrogen oxide values in the exhaust tailpipe region, and if the nitrogen oxide values are still deviated upward from the setpoint value after the actuation of the introduction device, performing a first adjustment of the internal combustion engine to regulate a regeneration of the LNT in order to provide a second correction of the nitrogen oxide values in the exhaust tailpipe region. The method further includes, if the nitrogen oxide values are still deviated upward from the setpoint value after the first adjustment of the internal combustion engine, performing a second adjustment of the internal combustion engine to regulate an operating mode of the internal combustion engine in order to provide a third correction of the nitrogen oxide values in the exhaust tailpipe region.
The method is advantageous because the nitrogen oxide emissions can be controlled by various inter-coordinated devices which are associated with the exhaust tract. Furthermore, the nitrogen oxide emissions can be dynamically controlled over a relatively long operating period. After a correction of the nitrogen oxide emissions, the method can at any time be started again.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.